FIG. 16 is a block diagram showing in a conceptual fashion a technology to print color images using a color printer. A scanner 20 outputs to a computer 10 image data DT2 indicating a read image. The computer 10 displays the image on the CRT 22 based on the image data DT2 and causes the color printer 30 to print the image. Where it is desired to print the read image in color, R, G and B signals (hereinafter collectively termed ‘RGB signals’) indicating the amounts of red, green and blue in the image are used as the image data DT2.
In the computer 10, an application program 40 is run under the control of a prescribed operating system. CRT driver software 17 and printer driver software 41 are incorporated in this operating system. Image data DT1 to be relayed to the color printer 30 is output from the application program 40 via the printer driver software 41.
The application program 40 comprises photo editing software, for example, that performs image editing (retouching) of the image data DT2. The processing result DT3 obtained via the application program 40 is supplied to the CRT driver software 17 or the printer driver software 41.
When a print command is issued by the application program 40, the printer driver software 41 of the computer 10 converts the processing result DT3 into printing signals DT1 and sends them to the color printer 30. The color printer 30 contains various colors of ink, and the printing signals DT1 contain information regarding data indicating the dot formation status for multiple colors of ink (dot data) and regarding the amount of sub-scanning to be performed.
The printer driver software 41 incorporates a resolution conversion module 41a, a color conversion module 41b, a halftone module 41c, a rasterizer 41d and a color conversion module 41e. 
The resolution conversion module 41a converts the resolution of the processing result DT3 obtained from the application program 40 into a printing resolution in order to obtain a conversion result DT4. The conversion result DT4 naturally include color information. Based on the conversion result DT4, the color conversion module 41b uses the color conversion table 41e to determine the amount of each color of ink to be used by the color printer 30 for each pixel. The halftone module 41c performs so-called halftone processing. The rasterizer 41d arranges the dot data in the order of the data to be relayed to the color printer 30 and outputs the printing signals DT1 to the color printer 30 as final print data.
This technology has been introduced in Patent Document 1 (JP 2002-59571A), for example. Furthermore, a technology to print multiple images corresponding to multiple colors using a color printer has been introduced in Patent Document 2 (JP H11-196285A), for example.
The above technologies for displaying color images on a printing medium are widely used. However, monochrome images having a single color hue (also termed ‘monotone images’) exhibit a particular ‘feel’ when they have a prescribed color tone, and there is a strong demand for printing of monochrome images. The conventional technology shown in FIG. 16 can also print monochrome images.
For example, an image read by the scanner 20 is caused to be recognized by the computer 10 as an achromatic grey image. Because all pixels of a grey image have the same amounts of red, green and blue, the R, G and B signals of the image data DT2 all have the same value.
The application program 40 performs processing to assign a prescribed color tone to the grey image expressed by the image data DT2 (hereinafter termed ‘color tone assignment processing’) and generates a processing result DT3.
FIGS. 17 and 18 are graphs representing the conversion of RGB signals in accordance with the color tone assignment processing, and shows new R, G and B signals included in the processing result DT3 obtained via the color tone assignment processing as R′, G′ and B′ signals (hereinafter collectively termed R′G′B′ signals). The R, G and B signals of the image data DT2 all have equal values. Here, a situation is described in which the tone values of the RGB signals comprise 256 steps corresponding to the integers 0-255.
FIG. 17 shows a case in which it is desired to print a grey image as a grey image (hereinafter ‘neutral tone’); FIG. 18 shows a case in which it is desired to print the grey image as having a cool color tendency (hereinafter ‘cool tone’); FIG. 19 shows a case in which it is desired to print the grey image as having a warm color tendency (hereinafter ‘warm tone’); and FIG. 20 shows a case in which it is desired to print the grey image as a faded color image (hereinafter ‘sepia tone’).
The R′G′B′ signals obtained in this fashion undergo resolution conversion via the resolution conversion module 41a, whereupon they are converted to amounts of various inks to be used by the color printer 30 using the color conversion table 41e in the color conversion module 41b. The values of the R′G′B′ signals are maintained even after resolution conversion by the resolution conversion module 41a. 
FIG. 21 is a graph describing the technology by which the amounts of C, M, Y and K ink representing the colors of cyan, magenta, yellow and black to be used are set based on the R′G′B′ signals using the color conversion table 41e. Because the R′, G′ and B′ signals are mutually independent, the color conversion table 41e is expressed schematically as a three-dimensional cube. Here, a situation is shown in which the tone values comprise the 256(=28) levels of 0-255. In order to limit the required memory capacity, it is not preferred that the color conversion table 41e store 28×28×28 (approximately 16.78 million) sets of data. Therefore, the data storage positions in the color conversion table 41e are set in a discrete fashion as lattice points, one for every 17 tone values, for example. Here, one data set includes three types of data representing ink amounts for C, M and Y, for example. FIG. 21 shows a position T0 corresponding to the values r0, g0, b0 for the R′, G′ and B′ signals, respectively.
However, in general, it can occur that a lattice point corresponding to given values r0, g0, b0 does not exist. In such a case, multiple lattice points surrounding the position T0 are generally selected and the ink amounts corresponding to the position T0 are determined via interpolation using the ink amounts stored for the selected lattice points.
In the above configuration, it is not easy to set a color tone of a monochrome image to a desired color tone. 256 types of RGB signals are sufficient to express the image data DT2 representing a grey image. However, because the R′, G′ and B′ signals expressing a monochrome image have mutually different values, printing of a monochrome image will need the same type of color conversion processing which is performed for a color image. Furthermore, the time required for such processing increases dramatically due to the trial and error operations required to set the color tone of the monochrome image.